Trump already won Stormy case: White House

The White House says Donald Trump has already won a case against porn star Stormy Daniels.

US President Donald Trump has already won a legal case against adult film actress Stormy Daniels in arbitration, his spokeswoman says, a day after she sued seeking to invalidate a "hush agreement" she signed with Trump's lawyer.

Daniels, whose legal name is Stephanie Clifford, says in the lawsuit Trump never signed the non-disclosure agreement, which was designed to keep her quiet about an "intimate relationship" she said she had with him more than a decade ago.

Trump's failure to sign renders the pact invalid, the lawsuit says.

Clifford has said she was taking legal action to be able to speak freely about the "intimate relationship" without fear of retribution.

According to the lawsuit, Clifford and Trump had an intimate relationship that began in mid-2006, not long after Trump's wife, Melania, gave birth to their son, and lasted well into 2007.

White House spokeswoman Sarah Huckabee Sanders denied Trump had an affair with Clifford and said arbitration with Clifford was won by the president's lawyers.

"The president has addressed these (allegations by Clifford) directly and made very well clear that none of these allegations are true," Sanders said.

"This case has already been won in arbitration and anything beyond that, I would refer you to the president's outside counsel."

Clifford's lawsuit says Trump lawyer Michael Cohen initiated a "bogus arbitration proceeding" against her on February 27, without notifying her, as part of efforts to "shut her up" and "protect Mr Trump".

Cohen's lawyer, Lawrence Rosen, said on Wednesday that Cohen obtained an order from an arbitrator prohibiting Clifford from disclosing confidential information related to the non-disclosure agreement.

Cohen has previously acknowledged he paid Clifford $US130,000 ($A182,000) in October out of his own pocket not to discuss her claims of an affair with Trump.

Asked about the payment by Cohen to Clifford, Sanders said she was not aware Trump knew of it at the time.